This invention relates generally to photolithography and more particularly to a method and system for varying the transmission of an attenuated phase shift photomask.
Integrated circuits are used as a basic design part in many varied applications ranging from radios and alarm clocks to high performance computers. The fabrication of these integrated circuits involves the creation of electronic circuits on semiconductor wafers. This process generally includes a series of procedures which maybe categorized into four basic operations. These operations are layering, patterning, doping and heat treating.
Patterning is the process of creating a pattern on a wafer, generally accomplished through photolithography and etching. Photolithography involves the transfer of a pattern from a mask to a wafer with light. This process is completed in a device referred to as a stepper, or scanner. A stepper includes a light source and a mask, in addition to other components. A mask for use in the stepper is fabricated with a desired pattern that corresponds to features to be formed on the wafer. A layer of photoresist, which is a material that is sensitive to light, is applied to the wafer. Light is passed through the mask and impinges on the photoresist in accordance with the pattern of the mask, thereby altering the exposed photoresist. The altered photoresist is then removed by development, while the unaltered photoresist remains as a protective layer over the wafer. Thus, parts of the wafer which require further processing are made available, while the remaining parts of the wafer are protected.
While some applications allow the use of masks which completely block the light from passing through, others require attenuated phase shift masks which allow a relatively small percentage of the light to pass through. A conventional attenuated phase shift mask allows a single, specific percentage of light to pass through. The desired percentage of light transmission through an attenuated phase shift mask for a particular application is experimentally determined by fabricating and testing several different masks, each with a unique transmission percentage. Typically only three or four masks are fabricated for testing due to the extremely high cost of fabrication. Thus, because only a few masks are available for testing due to their expense, it is difficult to identify precisely an optimum transmission percentage.
In accordance with the present invention, a method and system for varying the transmission of a single attenuated phase shift mask are provided that substantially eliminate or reduce disadvantages and problems associated with previously developed attenuated phase shift masks. In particular, an attenuated phase shift mask is constructed from a material with a transmission percentage that may be controlled by application of a force.
In one embodiment of the present invention, a system for varying the transmission of an attenuated phase shift mask is provided that includes an attenuated phase shift mask, a transparent support and a modulation system. The mask comprises a variable transmission material. The transparent support supports the mask. The modulation system is operable to apply a variable force at the mask to control the transmission of the mask.
Technical advantages of the present invention include providing an improved attenuated phase shift mask. In particular, the attenuated phase shift mask comprises a variable transmission material that is capable of changing its transmission percentage with the application of an outside force. As a result, a single mask may be fabricated for a particular application. The optimum transmission percentage may be determined by varying the transmission percentage of the mask. Thus, the cost is substantially reduced by eliminating the requirement of fabricating a plurality of masks. Additionally, an optimum transmission percentage may be determined, as opposed to simply choosing a transmission percentage based on the better results obtained from a few discrete transmission percentages.
Other technical advantages will be readily apparent to one skilled in the art from the following figures, description, and claims.